A Radio Frequency (RF) Identification (RFID) system generally includes a reader and an RFID tag. The reader generates a magnetic field to power the RFID tag, usually at frequency of 13.56 MHz or 125 kHz. An RFID antenna is inductively coupled to the reader antenna. The RFID tag contains a power rectifier to convert the varying magnetic field received through the antenna to a Direct Current (DC) voltage source that powers the RFID tag. The rectified voltage signal may be further regulated to a lower voltage (e.g., around 1.8 to 2.4V) to power digital circuits required for RFID tag operation.
Some RFID tags require nonvolatile memory, usually an EEPROM array. The EEPROM array interfaces to the digital circuits and typically shares the same voltage supply. The EEPROM array requires a charge pump to generate a high voltage DC level for programming EEPROM memory cells.
There are many conventional charge pump designs that can generate high voltage from a low voltage DC source. These conventional charge pumps, however, are inefficient at lower source voltages. Accordingly, an EEPROM program operation is often the most power consuming task for RFID tags due to the inefficient charge pumps.
Conventional charge pumps for nonvolatile memories use internal chip voltage and internally generated clock signals to produce high voltage. As technologies evolved, the internal chip voltages have been steadily lowered. Because the high voltage required for nonvolatile memory programming has not scaled proportionately, the size and power requirements of the charge pump have increased. This problem is compounded on RFID products because increased power requirements can reduce the RFID tag operating range. When the charge pump is turned on in a weak RF field, the internal voltage may collapse, causing an illegal write, or the sudden increase in current load may interfere with normal RFID communications.